1. Field of the Invention
The present invention relates generally to an alkaline storage battery such as a nickel-metal hydride battery, a nickel-cadmium battery, or a nickel-zinc battery and a nickel electrode for an alkaline storage battery employed as a positive electrode of such alkaline storage battery and is particularly characterized in that the nickel electrode for an alkaline storage battery formed by applying a paste containing active material particles composed of nickel hydroxide to a conductive substrate and drying said paste is modified to improve charge/discharge cycle performance of the alkaline storage battery under high temperature conditions.
2. Description of the Related Art
An alkaline storage battery such as a nickel-metal hydride battery or a nickel-cadmium battery has conventionally employed as its positive electrode a nickel electrode for an alkaline storage battery in which nickel hydroxide is used as an active material.
Conventionally, in such nickel electrode for an alkaline storage battery, a sintered nickel electrode formed by impregnating with nickel hydroxide as the active material a sintered substrate which is obtained by filling a porous steel sheet and the like as substrate with nickel powder and sintering said substrate has been used because conductivity of the nickel hydroxide used as the active material is low.
However, in such sintered nickel electrode, bond between particles of the nickel powder is weak. Accordingly, the nickel powder drops out easily when the substrate having high degree of porosity is used. Therefore, the maximum porosity of such substrate is 80% in actual conditions, and hence, the substrate is not sufficiently filled with nickel hydroxide as the active material, thus an alkaline storage battery having a large capacity was hardly attained.
Further, in the above-mentioned sintered nickel electrode, substrate including the porous steel sheet is used, thus filling density of the active material is generally small. In addition, a pore diameter of the nickel powder formed by sintering is generally small, for example, not more than 10 xcexcm. Thus, in filling the substrate with the active material, solution impregnating method in which laborious work is repeatedly performed must be taken, thereby degrading productivity.
Therefore, a paste type nickel electrode for an alkaline storage battery formed by applying a paste which is obtained by mixing the active material particles composed of nickel hydroxide with a aqueous solution of a binding agent such as methyl cellulose to a conductive substrate having the high degree of porosity such as foamed nickel and drying said paste has been used.
In such paste type nickel electrode for an alkaline storage battery, the conductive substrate having the porosity of not less than 95% can be used. Accordingly, the conductive substrate can be filled with a large number of active materials, thus, the alkaline storage battery having the large capacity is attained, and the conductive substrate can be easily filled with active materials thereby improving the productivity.
However, in such paste type nickel electrode for an alkaline storage battery, when the conductive substrate having the high degree of porosity is used to fill the conductive substrate with the large number of active materials, collecting current of the conductive substrate degrades, thereby reducing the utilization of the active materials.
Therefore, in recent years, in such paste type nickel electrode for an alkaline storage battery, a method in which metal cobalt or a cobalt compound composed of a cobalt oxide or a hydroxide as a conductive agent are added to the above-mentioned active material particles composed of nickel hydroxide, subsequently the above-mentioned metal cobalt and the cobalt compound are oxidized to xcex2-CoOOH which is cobalt oxyhydroxide by charging, to increase the conductivity of the electrode, thus to improve the utilization of the active materials has been used.
However, even in a case in which the metal cobalt or the cobalt compound as the conductive agent are added to the active material particles composed of nickel hydroxide, there still have remained problems that when the paste type nickel electrode for an alkaline storage battery is employed as the positive electrode of the alkaline storage battery and is charged under high temperature conditions, an oxygen overvoltage in the positive electrode is decreased, thus in addition to a charge reactivity in which nickel hydroxide is oxidized to nickel oxyhydroxide, a side reaction in which an oxygen evolution reactivity occurs and hence, charge characteristics is decreased occur.
Therefore, in Japanese Patent Laid-Open No. Hei8(1996)-222213, in the paste type nickel electrode for an alkaline storage battery, adding a niobium compound and the like in addition to the conductive agent composed of the metal cobalt or the cobalt compound to the surface of the active material particles composed of nickel hydroxide to increase the oxygen overvoltage in the positive electrode by the niobium compound and the like, and hence to improve the charge characteristics under high temperature conditions has been proposed.
However, even in the case where the paste type nickel electrode for an alkaline storage battery in which the niobium compound and the like in addition to the conductive agent composed of the metal cobalt and the cobalt compound are added to the surface of the active material particles composed of nickel hydroxide is used, when charge/discharge is carried out under high temperature conditions, discharge depth during discharge becomes deep, thus cobalt oxyhydroxide which is oxidized metal cobalt or oxidized cobalt compound is reduced to the cobalt hydroxide, then the cobalt hydroxide dissolves in an alkaline electrolyte solution of the alkaline storage battery, subsequently, the cobalt hydroxide deposits on the surface of the active material particles.
A speed in which the cobalt hydroxide dissolves in the alkaline electrolyte solution and deposits as mentioned above is so fast that when the charge and discharge is carried out repeatedly under high temperature conditions, cobalt hydroxide does not deposit on the surface of the active material particles composed of nickel hydroxide uniformly but the cobalt hydroxide segregates on the surface of the active material particles, and a part of the cobalt hydroxide diffuses in the pore, thereby reducing the conductivity of the nickel electrode for an alkaline storage battery gradually, as the result, a charge/discharge cycle performance under high temperature conditions is degraded.
An object of the present invention is to modify a nickel electrode for an alkaline storage battery formed by applying a paste containing active material particles composed of nickel hydroxide to a conductive substrate and drying said paste.
Another object of the present invention is, in an alkaline storage battery employing as its positive electrode the above-mentioned nickel electrode for an alkaline storage battery, to prevent a discharge capacity from decreasing when charge/discharge is carried out under high temperature conditions, and to improve a charge/discharge cycle performance under high temperature conditions.
The nickel electrode for an alkaline storage battery according to the present invention is a nickel electrode for an alkaline storage battery formed by applying the paste containing the active material particles composed of the nickel hydroxide to the conductive substrate and drying said paste, wherein a conductive layer consisting of cobalt oxide containing sodium is formed on a surface of said active material particles, and niobium powder and/or niobium compound powder is added to the surface of said active material particles.
As the above-mentioned nickel electrode for an alkaline storage battery, when the conductive layer consisting of the cobalt oxide containing sodium is formed on the surface of the active material particles composed of the nickel hydroxide, collecting current in the electrode is improved, thereby improving the utilization of the active material because electric conductivity of the cobalt oxide containing sodium is higher than that of metal cobalt or cobalt compound.
Further, as the above-mentioned nickel electrode for an alkaline storage battery, when the conductive layer consisting of the cobalt oxide containing sodium is formed on the surface of the active material particles composed of nickel hydroxide and the niobium powder or the niobium compound powder is added to the surface of the active material particles composed of the nickel hydroxide, the cobalt oxide containing sodium is prevented from being reduced to the cobalt hydroxide during discharge and dissolving into an alkaline electrolyte solution in the alkaline storage battery even in a case in which the alkaline storage battery employing the nickel electrode for an alkaline storage battery is charged/discharged under high temperature conditions. In addition, even in the case in which a part of the cobalt oxide containing sodium is reduced to the cobalt hydroxide, the speed in which the cobalt hydroxide dissolves in the alkaline electrolyte solution and deposits is delayed, the cobalt hydroxide is prevented from segregating on the surface of the active material particles and a part of the cobalt hydroxide is restrained from diffusing in the pore, as the result, the charge/discharge cycle performance under high temperature conditions is improved by an effect of the above-mentioned niobium or the niobium compound.
In the above-mentioned nickel electrode for an alkaline storage battery, the conductive layer consisting of the cobalt oxide containing sodium is formed on the surface of the active material particles composed of the nickel hydroxide by mixing metal cobalt powder, cobalt hydroxide powder, cobalt monoxide powder, or cobalt oxyhydroxide powder with the active material particles, or by forming a layer consisting of metal cobalt, cobalt hydroxide, cobalt monoxide, or cobalt oxyhydroxide on the surface of the active material particles, in both cases followed by adding a sodium hydroxide aqueous solution and heat-treating in the atmosphere that is in the presence of oxygen at a temperature of 50 to 200xc2x0 C.
In heat-treating, the temperature is set in the range of 50 to 200xc2x0 C. because in the case in which the temperature is not more than 50xc2x0 C., CoHO2 which is low in the electric conductivity deposits while in the case in which the temperature is not less than 200xc2x0 C., 3-cobalt tetraoxide Co3O4 which is low in the electric conductivity deposits, accordingly in both cases, the conductive layer having a high conductivity is not attained. When the particles of the cobalt oxyhydroxide are added to the surface of the active material particles or the layer composed of the cobalt oxyhydroxide is formed on the surface of the active material particles, CoHO2 does not deposit even in the case in which the heat-treating temperature is not more than 50xc2x0 C. However, sodium is hardly contained, accordingly the conductive layer having the high conductivity is not attained. Time for the above-mentioned heat-treating is not especially limited but is altered appropriately depending on concentration of the sodium hydroxide to be used or the heat-treating temperature. The time is approximately set in the range of 0.5 to 10 hours.
When the conductive layer consisting of the cobalt oxide containing sodium is formed on the surface of the active material particles composed of the nickel hydroxide as mentioned above, the cobalt oxide containing sodium contains a quite high electric conductivity, although its chemical structure is not clear. Accordingly, the cobalt oxide containing sodium is expected to be not a mixture of the cobalt oxide and sodium but a intercalation complex in which sodium is inserted into a crystal of the cobalt oxide.
The above-mentioned layer consisting of metal cobalt, cobalt hydroxide, or cobalt monoxide is formed on the surface of the active material particles composed of the nickel hydroxide by mechanical charging method in which metal cobalt powder, cobalt hydroxide powder, or cobalt monoxide powder is added to the nickel hydroxide powder and dry mixing said nickel hydroxide powder by a compressible crusher under inert-gas atmosphere.
The above-mentioned layer consisting of cobalt hydroxide is formed on the surface of the active material particles composed of nickel hydroxide by depositing cobalt hydroxide on the surface of the nickel hydroxide particles including the steps of adding nickel hydroxide powder to a cobalt salt aqueous solution such as cobalt nitrate, dropping an alkaline aqueous solution such as a sodium hydroxide aqueous solution into an obtained mixture while agitating the obtained mixture to adjust the pH of the solution to around 11 and reacting a resultant solution for an appointed time while agitating the resultant solution.
The above-mentioned layer consisting of the cobalt oxyhydroxide is formed on the surface of the active material particles composed of nickel hydroxide, for example, by oxidizing the cobalt hydroxide including the steps of forming the layer consisting of cobalt hydroxide on the surface of the active material particles composed of nickel hydroxide, and reacting the layer thus formed with hydrogen peroxide water which is heated to about 40xc2x0 C.
In forming the conductive layer consisting of the cobalt oxide containing sodium on the surface of the active material particles composed of nickel hydroxide as mentioned above, when the weight ratio of the conductive layer is too small, the conductivity of the nickel electrode for an alkaline storage battery is not fully improved. On the other hand, when the weight ratio of the conductive layer is too large, a ratio of nickel hydroxide in the nickel electrode for an alkaline storage battery is decreased, thereby decreasing the discharge capacity. Therefore, the weight ratio of cobalt element in the conductive layer based on the weight ratio of the active material particles composed of nickel hydroxide is preferably set in the range of 1.0 to 10 wt %.
In the above-mentioned conductive layer consisting of the cobalt oxide containing sodium, when the weight ratio of sodium element in the cobalt oxide containing sodium is too small or too large, the cobalt oxide containing sodium is easily reduced to cobalt hydroxide during discharge under high temperature conditions in both cases. Therefore, the weight ratio of the sodium element in the cobalt oxide containing sodium is preferably set in the range of 0.1 to 10 wt %.
In adding the niobium powder or the niobium compound powder to the surface of the active material particles on which the above-mentioned conductive layer is formed, when an additive weight ratio is too small, the charge and discharge cycle performance under high temperature conditions is not fully prevented from decreasing. On the other hand, when the additive weight ratio is too large, the ratio of nickel hydroxide in the nickel electrode for an alkaline storage battery is decreased, thereby decreasing the discharge capacity. Therefore, the weight ratio of a niobium element in the niobium powder and/or the niobium compound powder to be added based on a total weight of the active material particles composed of the nickel hydroxide and the above-mentioned conductive layer is preferably set in the range of 0.2 to 4.0 wt %.
Examples of the above-mentioned niobium compound include Nb2O5, Nb2O3, NbO, NbO2, NaNbO3, LiNbO3, KNbO3, Nb2O5. xH2O.
When a particle diameter of the above-mentioned niobium powder or the niobium compound powder is too large, an area of the niobium powder or the niobium compound powder which contacts with the surface of the active material particles on which the conductive layer is formed is decreased, thus sufficient effect is not attained. Therefore, the niobium powder and/or the niobium compound powder having an average particle diameter of not more than 100 xcexcm is preferably used.
In the nickel electrode for an alkaline storage battery according to the present invention, at least one type of an element selected from a group consisting of zinc, cobalt, calcium, magnesium, aluminum, manganese, yttrium, and ytterbium is preferably incorporated into the above-mentioned active material particles composed of the nickel hydroxide and the ratio of these elements based on the total weight of the nickel in the above-mentioned nickel hydroxide and these elements is preferably set to not more than 10 atomic % to prevent the potassium ion and the like in the alkaline electrolyte solution from being intercalated into the crystal of nickel hydroxide as the active material for the effect of the elements incorporated, thus to prevent the decrease of the charge/discharge capacity by drying out of the alkaline electrolyte solution. Especially, when at least one of the elements selected from zinc and cobalt is incorporated, the decrease of the charge/discharge capacity by drying out of the alkaline electrolyte solution is further prevented because of a greater effect of these two elements.
In the nickel electrode for an alkaline storage battery according to the present invention, when at least one type of element powder or its compound powder selected from the group consisting of yttrium, ytterbium, calcium, aluminum, erbium, gadolinium, thulium, lutetium, and zinc in addition to the niobium powder and/or the niobium compound powder are added to the surface of the active material particles on which the conductive layer consisting of cobalt oxide containing sodium is formed as mentioned above, the charge/discharge cycle performance under high temperature conditions is further improved. Especially, when yttrium or yttrium compound are added, the charge/discharge cycle performance under high temperature conditions is remarkably improved because of the greater effect.
In the alkaline storage battery employing as its positive electrode the nickel electrode for an alkaline storage battery according to the present invention, examples of the conductive substrate to apply the above-mentioned paste containing the active material particles include foamed nickel, pannose metal fiber, punching metal, and the like.
In the nickel electrode for an alkaline storage battery according to the present invention, the alkaline electrolyte solution containing potassium, lithium, and sodium is preferably used to improve discharge performance under high temperature conditions, thus to prevent an oxygen evolution during charge/discharge, and more preferably, the alkaline electrolyte solution containing 4.0 to 10.0 mol/l of potassium hydroxide, 0.1 to 2.0 mol/l of lithium hydroxide, and 0.2 to 4.0 mol/l of sodium hydroxide.
Examples of the alkaline storage battery employing as its positive electrode the above-mentioned nickel electrode for an alkaline storage battery include a nickel-metal hydride battery employing as its negative electrode a hydrogen absorbing alloy electrode, a nickel-cadmium battery employing as its negative electrode a cadmium electrode, and a nickel-zinc battery employing as its negative electrode a zinc electrode.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.